"I'd rather have a hub motor but have a small battery pack affixed elsewhere, maybe in a bottle cage, where it doesn't have to rotate."
It's not as big a penalty as you think. The weight is nearer the center of the wheel. This design has real packaging advantages over what you describe. It's also not new.
"20 MPH can be sustained by a fit rider who isn't elderly."
20 MPH can be sustained by a fit rider who is elderly.
Electric assist isn't needed for fit riders and bumping the speed to 25 MPH wouldn't make it better for its purpose. Bike commuters are not well served by devices designed to increase their riding speeds, they are well served by devices that expand the range for which cycling is practical. When commuting the goal isn't to ride as fast as you can, it is to arrive safely while interacting appropriately with traffic and hopefully not flat constantly while doing so.
I am also 52 years old and I have no problem sustaining 20 MPH in the flats. On my 9 mile one way commute, it is simply not possible for me to complete the ride in less than 30 minutes without a big tailwind. I do not need electric assist at my distances but others might. I could consider longer distances with assist, though, and my average speeds would improve even with the 20 MPH limitation. I have no desire to ride at high speeds on the shoulders with cars coming at me oblivious to my existence. I am forced to take emergency measures once every couple hundred miles typically. Safety is a far bigger issue than top speed.